1. Field of the Invention
The present invention relates generally to the measurement of films and more specifically to the simultaneous determination of the gauge and orientation of polymer films.
2. Description of the Prior Art
The present trend in the production of flexible packaging is toward multilayer polymer structures that contain a number of materials. The reason is that, quite often, no single material has the combination of physical and chemical properties required for the application. Furthermore, there is a trend away from the use of foil and paper, both of which require large amounts of energy to manufacture, and are consequently expensive. Polymer films are being used in their place.
Multilayer structures utilizing polymer films can be produced by lamination, coating or coextrusion processes. In coextrusion, a number of films are simultaneously extruded through a common die. This coextrusion process allows very thin layers of high performance, expensive polymers to be coextruded along with less expensive materials, to produce low cost, yet high performance, flexible packaging materials.
Coextrusions are used to make trash bags, meat wraps, shipping sacks, cheese and snack packaging, cereal liners and bakery overwrap, among other things. As an example, polyethelene (PE) and nylon are often used in combination in snack food bags, because nylon has good strength and toughness and is a good oxygen and carbon dioxide barrier, while PE is a good moisture barrier. In addition, PE carries printing ink and other coatings well.
In the manufacture of any multilayer polymer film, the thickness or gauge of the individual layers as well as the total thickness will vary as a function of process parameters. Thus, in order to manufacture a multilayer structure with adequate thickness in each layer, the average layer thickness will be made to exceed the minimum required so that thickness variations do not result in regions of inadequate thickness. If, however, thickness could be controlled, then less polymer would be required, on average. In lamination applications, individual layers exist prior to being integrated into the final multilayer structure, and therefore may be gauged independently by appropriate methods. In coating applications, layers are added one at a time, and may therefore be gauged by measuring the increased thickness of the product after each coating operation. However, this is not the case in coextruded structures. The polymer films are combined in the coextrusion die, and do not exist individually. Therefore, it is necessary to gauge the different materials simultaneously, in the presence of one another. It is desirable that such gauging be conducted on-line to minimize down time and reduce on-line waste. On-line gauging also facilitates closed-loop control of the process. It is also desirable that such gauging require little or no calibration by an operator because of time waste and the subjective nature of most operator intervention. In the same regard, it is desirable to gauge the thickness of a particular coextrusion under various conditions of overall gauge and relative composition without recalibration. The gauging method should be nondestructive and non-contact, of course.
There are three common on-line gauging methods: caliper, nuclear and infrared (IR). Caliper gauges are of contact and non-contact types. The majority of caliper gauges operate on the principles of magnetic reluctance or inductance. These methods are insensitive to compositional variations in the coextrusion, and, therefore, are unable to distinguish individual materials in a co-extrusion.
Nuclear gauges are non-contact. They operate by passing radiation (usually .beta. particles) through the film being gauged. Again, they are relatively insensitive to compositional variations, and are capable of measuring only total gauge. They are not appropriate for co-extrusions.
IR gauges (here inclusive of radiation of wavelength 1-50 microns) are non-contact. They operate by passing IR radiation through the sample. In contrast to the other methods, the IR technique can distinguish individual materials, since the transmission of IR radiation of given wavelength is a function of the material through which it passes. In general, the infrared transmittance of a film is given by EQU T(.lambda.)=e.sup.-.alpha.(.lambda.)d ( 1)
where d is the film thickness, .alpha.(.lambda.) is the absorption coefficient, and .lambda. is the wavelength of the infrared radiation. This simple relationship is known as Beer's Law. (See, for example, G. Barrow, "Introduction to Molecular Spectroscopy", McGraw Hill, N.Y. 1962). A more common mathematical form is found by taking logarithms of equation (1) to get EQU A(.lambda.)=.alpha.(.lambda.)d (2)
where A(.lambda.) is the absorbance, or the negative logarithm of transmittance T(.lambda.). For multilayer films, e.g. coextrusions, Beer's Law can be applied n times for n materials if the absorption bands do not overlap in the n materials. Such a situation rarely exists in practice. A more sophisticated and realistic approach accounts for spectral overlap by solving simultaneously for the absorptions of each of n materials at the n wavelengths. This multivariate form of Beer's Law is given by: ##EQU1## Here, n is the number of materials, .lambda..sub.i is the wavelength chosen that is most sensitive to changes in thickness of the i.sup.th material, .alpha..sub.i (.lambda..sub.j) is the absorption coefficient of the i.sup.th material at the j.sup.th wavelength, and d.sub.i is the thickness of the i.sup.th material. In equation (3), the absorbances A(.lambda..sub.i) are measured and the absorption coefficients .alpha..sub.i (.lambda..sub.j) are known. The equations are then solved for the thicknesses d.sub.i. It should be noted that often the absorbance A(.lambda..sub.i) is replaced by an absorbance difference .alpha.(.lambda..sub.i)-A(.lambda..sub.i ') in order to account for baseline variations.
The absorption of IR radiation by polymer molecules results from the interaction between changing dipole moments in the material and the electric vector of incident radiation. In fact, the absorption coefficient is proportional to the square of the dot product of the change in dipole moment d.mu. and the electric vector E of the electromagnetic radiation. This dot product is given by EQU d.mu..multidot.E=(d.mu.)(E) cos .phi., (4)
where .phi. is the angle between them. In an isotropic medium, the absorption coefficient .alpha..sub.i (.lambda..sub.j) is independent of the direction of propagation of the radiation. In the case of anisotropic media, however, .alpha..sub.i (.lambda..sub.j) is more properly designated as .alpha..sub.i.sup.k (.lambda..sub.i).sub.k=x, y z where x, y and z are the measurement axis directions. Additionally .alpha..sub.i (.lambda..sub.j) can change if the orientation of the medium is changed as a result of applied stresses. This orientation results in alignment of the long macromolecular chains that comprise the polymer film. The chains tend to line up in the direction of applied stress. Groups that are part of the chain backbone and pendant on the backbone will move along with the chain without serious deformation, at least to a first approximation. Thus, the position, relative to the measurement axis system, of the dipoles responsible for the vibrational transitions that give rise to absorption of infrared light will change. The interaction between these changing dipoles and the electric vector of incident radiation will, of course, also change and thus the absorption coefficient will change.
It is well known that the blown-film extrusion process imparts stresses to the film. Starting in the die, the molten plastic undergoes shear forces upon extrusion through the die lips. Upon being blown into a tube, the film walls have a force applied to them in a direction transverse to the direction of motion of the film. The magnitude of this force determines the diameter of the tube and hence the width of the film web. Additionally, the film experiences a force in the longitudinal or machine direction, exerted by the take up rollers. The film is stretched in the machine direction, thereby determining its gauge. Finally, the tube is slit or collapsed to form a web. These transverse and longitudinal stresses induce strain in the layers comprising the coextrusion. The stresses result in orientation of the long chain macromolecules. It is also important to note that the coextrusion is under stress even after being cut from the roll. Each layer is in intimate contact with two other layers, except for the outer layers which are contacted on one side only. These sandwich layers serve to keep the inner layer rigid. More importantly, the stress and corresponding strain are not fixed when the film is cut from the roll. The coextruded film undergoes stress relaxation and this relaxation changes the orientation of the macromolecules in each layer of the film. The film does not simply mechanically recover from the stress since stress is still being applied, as indicated above. Consequently, absorbances at the chosen wavelengths will change in time. Also, physical properties such as material strength and chemical properties such as permeability to gases such as oxygen, carbon dioxide and nitrogen, and liquids such as water, are related orientation. As the film relaxes, these properties change. It is therefore important that the manufacturer examine the product before shipment.
The fact that polymer films are oriented complicates the on-line gauging of such films. The infrared gauging technique requires knowledge of the absorption coefficient at each wavelength. Since the absorption coefficient varies with orientation, the standard infrared gauging techniques as expressed in equations (2) and (3), become inadequate because they incorrectly assume the constancy of .alpha..sub.i (.lambda..sub.j). It is also important to note that since orientation of polymer films affects their physical and chemical properties, it is also useful to be able to measure degree of orientation by an on-line, non-contact method.
There are a number of infrared techniques that have been used for measuring the gauge of polymer films. The simplest technique involves the measurement of the infrared absorption at a single wavelength (or wavelength pair), and calculation of thickness according to Beer's Law. Williams and Pugh (U.S. Pat. No. 4,027,161) describe a method of minimizing interference effects in such an instrument. Fumoto and Sawaguchi describe another solution to this problem (U.S. Pat. No. 4,429,225). A means of modulating the photodetectors in a infrared film gauging system is described by Ruskin (U.S. Pat. No. 3,825,755). An infrared sensor designed to measure coextruded films is described by B. Burk, in an article entitled "Infrared Sensors Control Thickness of Film-Coextrusion Plies," Modern Plastics, January 1984, pp. 84-8. This technique utilizes multiple wavelengths, but relies on non-overlap of absorption bands. This is a very restrictive technique, only applicable to a few special cases. Furthermore, it does not account for absorption coefficient changes resulting from orientation. The use of multiwavelength measurements with overlapping absorption bands is described in an articles entitled "New Sensors Tackle The Tough Jobs: Coextrusions, Foam, Filled Material", Modern Plastics, September 1979, pp. 88-91. Simultaneous equations are used in a multivariate analysis to measure the gauges of different polymers in a coextrusion. This technique does not account for orientation. The use of the multivariate approach in near IR instrumentation is described by R. Rosenthal, in "An Introduction to Near Infrared Quantitative Analysis," presented at the 1977 Annual Meeting of the American Association of Cereal Chemists. Reference is made to the measurement of the thickness of the layers in coextruded films. This technique, as stated previously, does not account for polymer orientation.
Two well-known techniques used to determine polymer orientation are dichroism and birefringence. The dichroic ratio is defined as the ratio of absorption of light polarized in a given direction to that polarized in the perpendicular direction (H. W. Siesler and K. Holland-Moritz, "Infrared and Raman Spectroscopy of Polymers," Marcell Decker, Inc. N.Y. (1980) p. 229). Dichroism requires anisotropy of absorption which in the IR region requires anisotropy of d.mu. for vibrational motion. Dichroism measurements on polymer films (Zbinden, "Infrared Spectroscopy Of High Polymers" Academic Press, N.Y. (1964), p. 212) are extremely useful for the determination of molecular orientation in polymers. However, the dichroic ratio, which is independent of thickness, cannot simultaneously determine orientation and film gauge.
Birefringence is defined as the difference between the refractive indices of a material as measured in two mutually perpendicular directions (Shurcliff "Polarized Light," Harvard University Press (1962), Chapter 7). Birefringence is associated with the anisotropy of the polarizability of the molecules comprising the material. Birefringence can be determined by placing the material between two polarizers whose major transmission axis are mutually perpendicular (crossed) and measuring the intensity of light that exits the crossed polarizers. If the material is not birefringent, no light exits the second polarizer. If it is, it renders the incident linearly polarized light elliptical by retarding the phase of one component of light relative to the other. This introduces a component of light perpendicular to that incident on the material. This perpendicular component exits the second polarizer. Birefringence measurements are used extensively to determine molecular orientation in polymer films (R. J. Samuels, "Structured Polymer Properties," John Wiley and Sons, N.Y. (1974) pp. 41-74). Since the birefringence is measured by using phase retardation, and phase retardation is proportional to thickness as well as birefringence, birefringence measurements themselves require independent means of determining sample thickness. Obviously, then, birefringence measurements alone cannot be used to determine film gauge.
It is therefore an object of this invention to provide a method of measuring the gauge of polymer films with infrared radiation, independent of the degree of orientation of the films. It is a further object of this invention to measure, simultaneously, the thickness of multiple polymer materials in a laminated, coated or coextruded structure, independent of the degree of orientation of each of the materials.
It is a further object of this invention to measure simultaneously, both the thickness and degree of orientation of polymer films.
It is a still further object of this invention to measure the degree of orientation of polymer films, independent of their thicknesses.
It is a still further object of this invention to measure, simultaneously, the thicknesses and degrees of orientation of polymer films, without the need for experimentally determining, for each production run, the value of the absorption coefficients.
It is a still further object of this invention to provide an on-line, non-contact method of measuring both the thickness and degree of orientation of polymer films.